Measuring instrument for use in orthopaedic surgery

ABSTRACT

A measuring instrument ( 2 ) for use during an orthopaedic surgical procedure to measure first and second distances from a reference point on a bone to first and second measurement points respectively. The measuring instrument has a first scale ( 24 ) for displaying the first distance and a second scale ( 28 ) for displaying the second distance. It includes a mask ( 26 ) which can be positioned against the second scale after the first distance has been measured in a position that is selected relative to the second scale dependent on the measured first distance, to restrict the length of the second scale that is visible to the user.

This invention relates to a measuring instrument for use in orthopaedicsurgery.

It is important for the success of a surgical procedure to replace anorthopaedic joint to determine as accurately as possible theconfiguration of the patient's natural joint. This informationfacilitates the correct selection of the components of a jointprosthesis for use during the procedure, and the correct positioning ofthe prosthesis components during the procedure. Correct selection andpositioning of the prosthesis components can help to ensure that thejoint provides the desired biomechanical performance after implantation.

Accurate determination of the configuration of a patient's joint isparticularly important in relation to the implantation of a knee jointprosthesis. For example, it can be important to determine (a) the sizeof the femur measured along the anterior-posterior axis, (b) theproximal extent of the femoral sulcus, and (c) the distance between thefemur and the proximal tibia, and the rotational orientation of thetibia relative to the femur, when the joint soft tissue is appropriatelytensioned. These factors can determine the selection of components whichare to be implanted in the knee replacement procedure, including thesize of the femoral component and the size of the spacer which is to beprovided between femoral component and the tibial component. The factorscan be inter-related in that, for example, a change in the size of thefemoral component that is selected will affect the distance between thatcomponent and the proximal face of the tibial component. This will havean effect on the tension in the joint soft tissue for a particular sizeof bearing or spacer component.

It is known to select the size of the femoral component of a knee jointprosthesis by measuring the distance from the posterior condylar surfaceto the anterior condylar surface. This measurement can be performedbefore the start of the joint replacement procedure using images of thefemur. However, it is generally desirable to measure this distanceduring the procedure. This can be done through the incision which ismade to access the joint by placing the knee in flexion.

The size of the bearing spacer component which is used between thefemoral component and the tibial component can also be selected beforethe start of the joint replacement procedure using images of the joint.Again, it is generally desirable to obtain additional distance dataduring the procedure. Measurements can be made with the knee in flexion.Additional measurements can be made with the knee in extension. Themeasurements of the distance between the femur and the tibia requiresthat the soft tissue extending between the femur and the tibia is placedin tension.

A preferred approach to measuring the femur in a knee replacementprocedure makes use of a sizing guide which has a pair of feet whichcontact the posterior condyles, and a stylus which contacts the anteriorcortex, extending beyond the femoral sulcus. The position of the styluscan be adjusted to adjust the point along the length of the anteriorcortex at which the anterior cortex is contacted by the stylus. Such asizing guide is disclosed in WO-A-2011/141722.

US-A-2007/162036 discloses a device which can be used to determine thesize of a femoral component and the size of a bearing component for usein knee replacement procedure. The device is located on a rod which isinserted into the intramedullary canal of the patient's femur. The sizeof the femoral component is measured in a first step by placing a pairof feet in contact with the femoral condyles while the knee is inflexion, and placing a stylus anteriorly in contact with the anteriorcortex. In a subsequent step, the plate which carries the pair of feetis moved away from the posterior condyles towards the tibia to place theligaments spanning the joint in tension. This can then be used tomeasure the space between the femur and the tibia. This information canbe used to ensure that an appropriately sized bearing component is usedin the implanted prosthesis, between the femoral and tibial components.

The present invention provides a measuring instrument for use during anorthopaedic surgical procedure to measure first and second distancesfrom a reference point on a bone to first and second measurement pointsrespectively, the measuring instrument having a first scale fordisplaying the first distance and a second scale for displaying thesecond distance, the instrument including a mask which can be positionedagainst the second scale after the first distance has been measured in aposition that is selected relative to the second scale dependent on themeasured first distance, to restrict the length of the second scale thatis visible to the user.

The invention also provides a measuring instrument for use during anorthopaedic surgical procedure to measure first and second distancesfrom a reference point on a bone to first and second measurement pointsrespectively, which comprises:

-   -   a. a reference structure which can be located relative to the        reference point on the bone,    -   b. a first measurement structure for measuring the first        distance between the reference point and the first measurement        point, the first measurement structure and the reference        structure being capable of movement relative to one another, and        the instrument including a first scale provided by a first scale        portion on the reference structure and a second scale portion on        the first measurement structure, the first scale indicating the        position of the first measurement structure relative to the        reference structure and so indicating the first distance,    -   c. a second measurement structure for measuring the second        distance between the reference point and a second measurement        point, the second measurement structure and the reference        structure being capable of movement relative to one another, the        instrument including a second scale which is provided by a third        scale portion on the second measurement structure and the first        scale portion, the second scale indicating the position of the        second measurement structure relative to the reference structure        and so indicating the second distance,    -   d. a mask which can be located against the first scale portion        in a position that is selected according to the size of the        first measurement, the mask restricting the length of at least        one of the first and third scale portions that is visible.

The instrument of the invention has the advantage that theinterpretation of measurement data when measuring the distance from thereference point to the second measurement point is made simpler becauseof the restriction on the length of the second scale that is visible tothe user. This can help to reduce the incidence of errors when using theinstrument.

The instrument can include a reference structure which can be locatedrelative to reference point, and at least one measurement structurewhich can be moved relative to the reference structure to a positionwhich correlates to one of the first and second measurement positions.When the instrument is for use in a knee replacement procedure tomeasure a patient's femur, the reference point might be the femoralanterior cortex or the posterior femoral condyles.

The reference structure can include a formation which can be positionedagainst the reference point to enable the reference structure to belocated relative to the reference point. By way of example, when thereference point is the femoral anterior cortex, the reference structuremight be a stylus which contacts the anterior cortex, extending beyondthe femoral sulcus. The position of the stylus can be adjusted to adjustthe point along the length of the anterior cortex at which the anteriorcortex is contacted by the stylus. By way of another example, when thereference point is a posterior femoral condyle, the reference surfacemight be a plate or other formation which can be positioned against theposterior condyle.

The measurement structure can include a plate member having a first facewhich is directed towards the reference structure and a second facewhich is directed away from the reference structure, the first faceforming part of the measurement structure. The first face can be used toposition the measurement structure relative to the first measurementpoint and the second face can be used to position the measurementstructure relative to the second measurement point. The instrument caninclude a driver for moving the plate member between the first andsecond measurement points. A plate member which is used in theinstrument of the invention can be a solid plate without openingsextending through it. This will be preferred for many applications.However, the plate might have one or more openings or hollows formed init so that, in some constructions, it can be seen as for example aframe.

The instrument can include (a) a first measurement structure formeasuring the first distance between the reference point and the firstmeasurement point, the first measurement structure and the referencestructure being capable of movement relative to one another, and (b) asecond measurement structure for measuring the second distance betweenthe reference point and a second measurement point, the secondmeasurement structure and the reference structure being capable ofmovement relative to one another.

The first and second measurement structures can have components incommon. Each of the measurement structures can include components suchas a housing for mounting the measurement structure, a driver which canbe operated to adjust the measurement structure, one or more scaleportions. The first and second measurement structures can have any ofthese components (and other components) in common.

The measurement instrument can include a plate member which has a firstface which is directed towards the reference structure and a second facewhich is directed away from the reference structure, the first faceforming part of one of the first and second measurement structures andthe second face forming part of the other of the first and secondmeasurement structures. The plate member can be fixed as part of themeasurement instrument (for example as a result of being formed with oneor more other parts of the instrument such as by a moulding or castingtechnique, or by a joining technique such as welding or brazing orsoldering, or by use of one or more fasteners such as screws or rivets),or it might be capable of be detached. This can facilitate use of theinstrument.

The measurement instrument can include (a) a first scale provided by afirst scale portion on the reference structure and a second scaleportion on the measurement structure, the first scale indicating theposition of the measurement structure relative to the referencestructure and so indicating the first distance, and (b) a second scalewhich is provided by a third scale portion on the measurement structureand the first scale portion, the second scale indicating the position ofthe measurement structure relative to the reference structure and soindicating the second distance. The mask can be used to restrict thelength of at least one of the first and third scale portions that isvisible to the user. It will be apparent that the first and secondscales share the first scale portion on the reference structure in thisconstruction.

The measurement structure can include a component for fixing themeasurement structure against movement relative to the bone in at leastone degree of freedom. In particular, it can be preferred that themeasurement structure is prevented from translating in the plane whichincludes the femoral anterior-posterior axis and the femoralmedial-lateral axis. It can be preferred that the measurement structurecan rotate at least through a limited angle about the femoralsuperior-inferior axis. This can enable the instrument to be alignedwith both medial and lateral posterior condyles.

This can allow a measurement structure to be used to measure the firstdistance, and to be adjusted to measure the second distance. Themeasurement structure can include at least one part which is used tomeasure the first distance and not the second distance, and at least onepart which is used to measure the second distance and not the firstdistance. For example, the measurement structure can include a platemember having a first face which is directed towards the referencestructure and which is used to measure the first distance and a secondface which is directed away from the reference structure and which isused to measure the second distance. The measurement structure caninclude the second scale portion which is used to measure the firstdistance and the third scale portion which is used to measure the seconddistance.

The measuring instrument can include an intramedullary rod which can befitted in the intramedullary canal of the patient's bone. Theintramedullary rod can be fixed to the measurement structure. Themeasurement structure can include a component by which it can be fixedto the intramedullary rod. Preferably, the rod is fitted in theintramedullary canal so that it will not move within the canal whensubjected during use of the instrument to forces which might otherwisecause it to move in translation and/or rotation.

The measurement structure can include (a) a formation for locating themeasurement structure relative to the first measurement point or thesecond measurement point, and (b) a driver for moving the formationrelative to the connector component. The formation can be provided by aplate member. A plate member which is used in the instrument of theinvention can be a solid plate without openings extending through it.This will be preferred for many applications. However, the plate mighthave one or more openings or hollows formed in it so that, in someconstructions, it can be seen as for example a frame.

The formation for locating the measurement structure relative to anintramedullary rod can comprise a socket in which the end of the rod canbe received. Preferably, the rod and the measurement structure can beconnected to one another such that relative movement between them isprevented, at least in some degrees of freedom, for example by means ofa clamp. The intramedullary rod might have a plate attached to it at itsend, and the formation on the measurement structure can then enable themeasurement structure to be fastened on to the plate. When the rod has aplate attached to it at its end, it will frequently be unnecessary forthe measurement structure to include a socket in which the end of therod can be received.

It can be preferred that the reference structure can be positioned onthe measurement structure relative to the reference point while themeasurement structure is connected against movement relative to thebone. This can be appropriate when the instrument includes a componentby which the measurement structure can be fixed against movementrelative to the bone in at least one degree of freedom. Accordingly, useof the instrument can involve fixing the measurement structure to thepatient's bone in a position to measure one of the first and seconddistances, and then positioning the reference structure on themeasurement structure so that it is appropriately located relative tothe reference point on the bone. For example, when the reference pointis on the anterior cortex on a patient's femur, the reference structurecan be positioned on the measurement structure such that a stylus on thereference structure contact the anterior cortex. Preferably, theinstrument includes a lock mechanism by which the reference structurecan be locked against movement relative to the measurement structure.

The measurement structure can include a housing having tower portion.The reference structure can have an opening formed in it in which thetower portion of the measurement structure is received. A lock mechanismcan comprise a collar on the reference structure which can be used tocompress a collet against the surface of the measurement structurehousing.

A driver can be used to adjust the measurement structure so that theposition of a formation by which the measurement structure is locatedrelative to a measurement point is adjusted relative to a connector forthe measurement structure. A convenient driver can be based on athreaded connection between a first threaded member which is connectedto the connector for the measurement structure, and a second threadedmember which can mate with the first threaded member, the secondthreaded member being connected to the formation by which themeasurement structure is located relative to a measurement point. Forexample, the driver can be based on a threaded connection between athreaded shaft, and a threaded bore in a rotatable knob.

The measurement structure can comprise an elongate housing. The housingcan be hollow along at least part of its length. A connector for fixingthe structure relative to a bone can be provided within the housing, forexample in the form of a socket in which the end of an intramedullaryrod can be received, or in the form of features by which the housing canbe attached to a plate or other structural component on the end of aintramedullary rod. The structure can include a driver knob which ismounted on the housing along the axis of the housing so that it canrotate. The axis of rotation is parallel to or coincident with the axisof the housing. The knob has a threaded bore within it. The bore can beopen at the end of the knob. This can facilitate cleaning and can meanthat the length of the bore does not restrict the range of movement ofthe shaft within the bore. The connector can have a shaft extending fromit which is threaded at one end so that the shaft can be received in thebore in the knob. When the connector is fixed to the bone, rotating theknob relative to the housing causes the shaft to translate relative tothe knob. This results in translation of the measurement structurehousing relative to the connector.

Preferably, at least one of the first and second scales comprises ascale portion on one component which has a single indicium and a scaleportion on another component which has a plurality of spaced apartindicia. The components (the reference structure and the measurementstructure) are capable of movement relative to one another. The indiciumon the other component that is adjacent to the single indicium on theone component indicates the measured distance. Preferably, the singleindicium that is used for the second scale in the second measurement isone of the plurality of spaced apart indicia of the first scale.

For example, the first scale can be provided by (a) a first scaleportion on the reference structure and (b) a second scale portion on themeasurement structure that is positioned relative to the firstmeasurement point, the first scale indicating the position of themeasurement structure relative to the reference structure and soindicating the first distance between the reference point and the firstmeasurement point. The first scale portion on the reference structurecan present a plurality of spaced apart indicia and the second scaleportion on the measurement structure can provide a single indicium. Thefirst distance is indicated by the indicium on the first scale portionthat is adjacent to the single indicium on the second scale portion.

The second scale can be provided by (a) a third scale portion on themeasurement structure that is positioned relative to the secondmeasurement point and (b) the first scale portion, the second scaleindicating the position of the measurement structure relative to thereference structure and so indicating the second distance between thereference point and the second measurement point. The third scaleportion on the measurement structure can present a plurality of spacedapart indicia and the first scale portion on the reference structure canprovide a single indicium, that being the indicium on the first scaleportion that was indicated in the first measurement step.

The mask can be positioned against the second scale in a position thatis selected relative to the second scale dependent on the measured firstdistance, to restrict the length of the second scale (the first scaleportion or the third scale portion or each of them) that is visible tothe user. This helps the user to interpret the measurement data that isprovided by the instrument. This can be particularly helpful when acommon component (especially the housing of a measurement structure)provides a scale portion which is used to perform the first measurementand a different scale portion which is used to perform the secondmeasurement. The mask can then be used to restrict the visibility of ascale portion (or a portion thereof) which is not used to perform thesecond measurement.

It can also be helpful when one of several indicia on a scale which isused to perform the first measurement is used as a selected indicium ona scale which is used to perform the second measurement. The mask canthen be used to identify the selected indicium, restricting thevisibility of other indicia on the scale which is used in the firstmeasurement and which are not selected for use in the secondmeasurement.

The mask can be fastened against the second scale in a plurality ofdifferent positions, each position corresponding to a different size ofthe first measured distance. For example, the reference structure andthe mask can fit together slidingly by means of cooperating rib andgroove features. For example, one of the reference structure and themask can have one or ribs formed on it, and the other can have one ormore grooves formed on it. The position of the mask on the referencestructure is determined by the choice of rib and groove.

The mask can be fastened against the second scale so that it can sliderelative to the second scale to allow it to be placed in a selectedposition corresponding to the size of the first measured distance.

The mask can be used to provide an indication to the user of thelocation of posterior references on the femur. For example, it can beused to provide an indication to the user of the location of theposterior cuts (through the posterior condyles) for the chosen size offemoral component of a knee joint prosthesis. Accordingly, the inventionprovides a kit for use in a surgical procedure which comprises:

(a) a measuring instrument according to the invention, in which the maskincludes at least one posterior cut reference surface, and(b) a femoral component of a knee joint prosthesis which includes ananteriorly facing bone contacting surface on a posterior condyle, inwhich the said bone contacting surface defines a posterior cut plane,in which the posterior cut reference surface when the mask is positionedon the second scale of the instrument appropriately having regard to themeasured first distance, lies in the posterior cut plane which isdefined by the femoral component when the femoral component isimplanted.

The invention also provides a kit for use in a surgical procedure toimplant a knee joint prosthesis, which comprises:

(a) a measuring instrument according to the invention, in which the maskincludes at least one posterior cut reference surface, and(b) a femoral component trial which includes an anteriorly facing bonecontacting surface on a posterior condyle, in which the said bonecontacting surface defines a posterior cut plane,in which the posterior cut reference surface when the mask is positionedon the second scale of the instrument appropriately having regard to themeasured first distance, lies in the posterior cut plane which isdefined by the femoral component trial when the femoral component trialis positioned on the resected femur.

Components of the instrument of the invention can be made from materialswhich are conventionally used in the manufacture of surgicalinstruments. Example of such materials include metals (for examplestainless steels) and polymers (for example polyolefins such aspolyethylenes and polypropylenes, polycarbonates, acetals and so on). Itcan be preferred that the measurement structure is made from a metal. Itwill frequently be preferred that the reference structure is made from ametal. The mask can be made from a polymer.

Embodiments of the invention are described below by way of example withreference to the accompanying drawings, in which:

FIG. 1 is a lateral view of a measuring instrument (without its mask),shown schematically in relation to a femur and a tibia as they might bepositioned in flexion during a surgical procedure to replace a kneejoint.

FIG. 2 is an anterior view of the measuring instrument shown in FIG. 1(with its mask).

FIG. 3 is an exploded view showing separated components of theinstrument shown in FIGS. 1 and 2.

FIG. 4 is a front view of the measuring instrument shown in FIG. 1(without its mask).

FIG. 5 is a view from behind of the mask which can be fitted to themeasuring instrument shown in FIG. 4.

FIG. 6 is an isometric view of the instrument when the plate member incontact with the femoral posterior condyles.

FIG. 7 is an enlarged view of the first scale on the instrument as shownin FIG. 6.

FIG. 8 is an isometric view of the instrument when the plate member incontact with the tibia.

FIG. 9 is an enlarged view of the first scale on the instrument as shownin FIG. 7.

The drawings show a measuring instrument which can be used in a surgicalprocedure to implant a knee joint prosthesis. Such a prosthesiscomprises a femoral component which is fitted to the femur and a tibialcomponent which is fitted to the tibia. The femoral component has aconvex bearing surface which provides medial and lateral bearingsurfaces corresponding to those provided by the anterior, distal andposterior surfaces of the medial and lateral condyles of the naturalknee. The tibial component generally presents a flat proximal surface. Abearing component is provided between the femoral and tibial components.The bearing component has medial and lateral concave recesses on itsproximal surface in which the medial and lateral condylar bearingsurfaces of the femoral component can be received and can articulate.The distal surface of the bearing component is planar so that thebearing component can slide on the tibial component, in rotation or intranslation or in both rotation and translation. Knee prostheses ofthese general types are well known.

It is common in knee replacement surgery to resect the tibia beforecutting the femur. The distal cut of the femur is frequently performedafter the tibial resection. The instrument that is provided by theinvention can then be used to ensure that the size of the femoralcomponent that is used is appropriate having regard to the size of thefemur. It can also be used to ensure that the bearing component that isused as the appropriate thickness having regard to (a) the size of thefemoral component and (b) the soft tissue which extends between thefemur and the tibia.

Referring to the drawings, FIGS. 1 and 2 show a measuring instrument 2that is in use in a surgical procedure to implant a knee jointprosthesis, illustrating the way in which the instrument is used in sucha procedure. The drawings show the tibia 4 which has been resectedproximally to define a flat surface 5 on to which the tibial component(not shown) of the prosthesis can be implanted, and the femur 6 whichhas been resected distally.

The instrument has a plate member 8 and a stylus 10. The instrument ismounted on an intramedullary rod 12 which is located in theintramedullary cavity in the femur. The instrument allows the positionof the plate member 8 to be moved relative to the intramedullary rod 12until the upper face 16 of the plate member is in contact with thefemoral posterior condyles 18. The instrument is shown in thisconfiguration in FIG. 1. As discussed below, the location of the tip ofthe stylus can be adjusted relative to the reference portion (on whichit is mounted) along the superior-inferior axis, and the location of thereference portion (with the stylus) can be adjusted relative to othercomponent of the instrument along the anterior-posterior axis. Theseadjustments allow the position of the stylus 10 to be adjusted until thetip of the stylus is in contact with the anterior cortex 20, beyond thefemoral sulcus 22.

The instrument provides a first scale 24 (see FIGS. 4 and 6) from whichinformation relating to the distance between the posterior condyles andthe anterior cortex, measured in a direction parallel to theanterior-posterior axis, can be obtained. As discussed below, this isprovided by locating a single indicium 104 on the measurement structureagainst a set 108 of indicia on the reference structure, where the setof indicia provides a first scale portion and the single indiciumprovides a second scale portion.

The position of the plate member 8 can be moved relative to theintramedullary rod 12 until the lower face 27 of the plate member is incontact with the flat face 5 of the resected tibia. The movement of theplate member can result in the soft tissue which extends between thetibia and the femur to be placed under tension. The instrument is shownin this configuration in FIG. 2.

FIG. 2 shows the instrument shown in FIG. 1 with its mask 26 attached.The mask provides a window 132 through which a limited length of asecond scale 28 (see FIG. 7) on the instrument is visible, whichprovides information relating to the distance between the anteriorcortex and the flat face of the resected tibia, measured in a directionparallel to the anterior-posterior axis, can be obtained. Thisinformation can be used to select a bearing component having anappropriate thickness having regard to the size of the femoralcomponent, and the soft tissue which extends between the femur and thetibia.

FIG. 3 shows separate components of the instrument. A measurementstructure 50 comprises a hollow elongate housing 52 which is hollow andhas a circular cross-section when viewed along its length. The housinghas upper and lower ends 53, 54. A pair of slots are formed in thehousing wall. The front slot 55 is visible in FIG. 3. A turret portion57 extends from the upper end 53 of the housing 52.

The hollow housing contains a connector block 56 which has a transversebore 58 extending through it. A shaft 60 extends through the housing 52and through the turret 57 along the housing axis from the connectorblock towards the upper end 53 of the housing. The shaft is threaded atthe end 61 which is remote from the connector block.

The measurement structure includes a driver assembly 62 which is mountedon the upper end 53 of the housing 52. The driver assembly includes adriver knob 75 and a driver sleeve 76. The driver sleeve fits over theturret on the housing so that the lower end of the driver sleeve sits onthe upper end of the housing. The turret is a close fit within thedriver sleeve so that the driver sleeve and the driver knob can rotateon the turret relative to the housing. The driver knob has a threadedbore 63 within it, the thread in the bore of the knob 62 cooperatingwith the thread on the end 61 shaft 60 so that, when the driver knob andthe driver sleeve are rotated relative to the housing and turret, theshaft is cause to move within the housing and the turret along thehousing axis. The bore can be open at the top end of the knob. This canfacilitate cleaning of the bore.

The measurement structure cooperates with an intramedullary rod 64 whichis a sliding fit in the transverse bore 58 in the connector block 56.The intramedullary rod extends through one or both of the slots in thewall of the housing when it is received in the transverse bore in theconnector block. The measurement structure can rotate around the axiswhich is defined by the intramedullary rod.

The measurement structure includes a plate member 66 which is fixed tothe elongate housing 52 at its lower end 54 when the instrument is inuse. The plate member has two lobes 68, 70 whose upper faces are shapedso as to replicate approximately the shape of the proximal face of thetibia, and which are intended to cooperate with the posterior faces ofthe medial and lateral femoral condyles respectively. It has upper andlower faces 72, 74 which are for contacting the posterior femoralcondyles and the proximal tibia respectively.

The instrument includes a reference structure 80 which comprises abridge portion 82 and medial and lateral limbs 84, 86. The bridgeportion has a circular hole 88 extending through it which is slightlybigger than the housing 52 of the measurement structure 50 so that thereference structure can be slid on to the housing of the measurementstructure.

The reference structure includes a locking collet 90 which comprises aplurality of flexible fingers 92 and a collar 94 which is threaded on tothe bridge portion of the reference structure. The fingers can bedeformed inwardly so as to grip the housing of the measurement structureby tightening the collar 94 down on to the bridge portion of thereference structure.

The reference structure includes a stylus 96 having a body portion 98which has a slot 100 formed in it. The stylus is mounted on a spigot onthe bridge portion of the reference structure so that it can sliderelative to the bridge portion, with the spigot sliding in the slot. Inthis way, the effective length of the stylus can be adjusted so that itextends just beyond the sulcus to the anterior cortex. This allows thetip of the stylus to be moved along the superior-inferior axis. Thestylus can include indicia 102 on the edges of the slot 100 to indicatethe lengths of the different sizes of femoral component that correspondto the different positions of the stylus.

The housing 52 of the measurement structure 50 bears a single indicium104, which is marked “SZ” in the drawings (for example see FIGS. 6 and7) which forms part of a first scale, and a set of indicia 106 whichform part of a second scale (which is discussed in more detail below).The set of indicia of the second scale is marked “mm” in the drawings(for example see FIG. 3). The reference structure 80 has a set ofindicia 108 (a first scale portion) in two groups on its medial andlateral limbs respectively which cooperate with the single indicium 104(a second scale portion) on the measurement structure to provide thefirst scale. The two groups of indicia on the reference structure aremarked “SZ” on each of the medal and lateral limbs. The first scale isused to obtain information concerning the distance between the posteriorcondyles and the anterior cortex, measured in a direction parallel tothe anterior-posterior axis, for use in selecting the appropriate sizeof a femoral component of the knee joint prosthesis. In the exampleshown in the drawings, the first scale indicates that the appropriatesize of femoral component having regard to the measuredanterior-posterior dimension is size 5.

As can be seen in particular in FIG. 4, the outside edges of the medialand lateral limbs 84, 86 of the reference structure are shaped with aplurality of parallel grooves 87 extending in a direction which isparallel to the intramedullary rod 64. The spacing between the groovescan (but need not) correspond to the spacing between the indicia 108 onthe reference structure.

FIGS. 3 and 5 show the mask component 120 of the instrument. The maskhas a front wall 122 and a pair of side walls 124, 126. The insidesurface of each of the side walls carries a series of inwardly extendingribs 127. The spacing between the ribs corresponds to the spacingbetween the grooves 87 in the outside edges of the limbs of thereference structure. The ribs are a sliding fit in selected grooves ofthe medial and lateral limbs of the reference structure so that the sidewalls of the mask can be slid on to the edges of the referencestructure.

The front wall 122 of the mask 120 is shaped to fit over the referencestructure 80 and the housing 52 of the measurement structure 50 when thereference structure is fitted on to the measurement structure.Accordingly, the front wall has a central portion 128 which is arcuate.It also has a slot 130 formed in it which is aligned with the front slot55 in the housing of the measurement structure when the mask is fittedto the reference and measurement structures, to accommodate the endportion of the intramedullary rod 64. The mask has a pair of windows 132formed in it. The windows are generally T-shaped (with the “T” turnedthrough 90° so that the limb of the “T” that is generally referred to asthe upright limb is horizontal and the limb of the “T” that is might bereferred as the cross limb is vertical). When the mask is fitted on tothe assembled reference and measurement structures with the ribs on theinside surfaces of the side walls of the mask are a sliding fit inselected grooves in the outside edges of the medial and lateral limbs ofthe reference structure, the horizontal limb of each of the windowsexposes a selected indicium from the set of indicia 108 (the first scaleportion) on the reference structure, and exposes one of the indicia fromthe set of indicia 106 (a third scale portion) on the measurementstructure 50. This can be seen in FIGS. 2 and 9, which show that theappropriate size of bearing component as 5 mm.

The instrument that is shown in the drawings has a first scale 24 whichprovides indicia corresponding to ten different sizes of femoralcomponent. The distance between the anterior cortex and the posteriorcondyles differs between consecutive sizes of component differs by 3 mm.The distance between adjacent grooves 87 on the outside edges of thelimbs of the reference structure is 3 mm.

The instrument that is shown in the drawings has a second scale whichprovides for bearing components to be used which have thicknessesbetween 5 and 22 mm. The second scale 28 indicates bearing componentthickness increments of 1 mm for bearing component thicknesses from 5 to8 mm, and bearing component thickness increments of 2 mm for bearingcomponent thicknesses from 8 to 22 mm.

The instrument that is shown in the drawings can be used in a surgicalmethod to replace a knee, which includes the following steps:

-   1. Perform the proximal tibial cut to form the planar proximal    surface 5 on the tibia on which a tibial component of a knee joint    prosthesis can be seated.-   2. Perform the distal cut on the femur.-   3. Insert the intramedullary rod 64 in the intramedullary cavity of    the femur 6.-   4. Locate the measurement structure 50 on the intramedullary rod 64    by positioning the end of the rod through the slots 55 in the wall    of the housing 52 and through the hole 58 in the connector block 56,    so that the measurement structure housing is pressed against the    distal face of the resected femur.-   5. Turn the knob 62 on the measurement structure (in the direction    indicated by the “SZ arrow”—see FIG. 2) to draw the plate member 66    upwardly towards the femoral posterior condyles until the upper face    72 of the plate member is in contact with the posterior condyles.-   6. Slide the reference structure 50 on to the measurement structure    50 so that the measurement structure housing 52 extends through the    hole 88 in the bridge portion 82 and through the collar 94,    effectively moving the tip of the stylus along the    anterior-posterior axis until the tip of the stylus 96 touches the    femoral anterior cortex, adjusting the length of the stylus as    necessary by sliding it along the anterior-posterior axis relative    to the bridge portion.-   7. Note the indicated size of the femoral component using the first    scale provided by the single indicium 104 on the measurement    structure 50 and the set 108 of indicia on the reference structure    80.-   8. Fit the mask 120 to the measurement and reference structures 50,    80 by sliding the ribs on the mask into the grooves 87 on the    reference structure, the grooves being selected so that the femoral    component size as indicated on the scale portion on the reference    structure appears towards the outward end of the horizontal limb of    one of the windows 130 in the mask.-   9. Turn the knob 62 on the measurement structure 50 (in the    direction indicated by the “mm arrow”—see FIG. 2) to move the    measurement structure including the plate member 66 downwardly    towards the resected tibia until the lower face 74 of the plate    member is in contacted with the tibia, and until the soft tissue    extending between the tibia and the femur is placed under    appropriate tension.-   10. Ensure that the tip of the stylus 98 touches the femoral    anterior cortex.-   11. Note the size of the bearing component as indicated towards the    inward end of the horizontal limb of the mask window which shows the    femoral component size in towards its outward end.-   12. Assess the location of the posterior cuts (through the posterior    condyles) for the chosen size of femoral component of a knee joint    prosthesis using the posterior edges of the mask.-   13. Tighten the collar 94 on the reference structure to prevent    relative sliding movement between the reference structure and the    measurement structure housing.-   14. Insert pins through the vertical limbs of the mask window and    into the holes in medial and lateral limbs (84, 86) of reference    structure to provide location for cutting block.

1. A measuring instrument for use during an orthopaedic surgicalprocedure to measure first and second distances from a reference pointon a bone to first and second measurement points respectively, themeasuring instrument having a first scale for displaying the firstdistance and a second scale for displaying the second distance, theinstrument including a mask which can be positioned against the secondscale after the first distance has been measured in a position that isselected relative to the second scale dependent on the measured firstdistance, to restrict the length of the second scale that is visible tothe user.
 2. The measuring instrument of claim 1, which includes areference structure which can be located relative to reference point,and at least one measurement structure which can be moved relative tothe reference structure to a position which correlates to one of thefirst and second measurement positions.
 3. The measuring instrument ofclaim 2, in which the measurement structure includes a plate memberhaving a first face which is directed towards the reference structureand a second face which is directed away from the reference structure.4. The measuring instrument of claim 2, which includes (a) a first scaleprovided by a first scale portion on the reference structure and asecond scale portion on the measurement structure, the first scaleindicating the position of the measurement structure relative to thereference structure and so indicating the first distance, and (b) asecond scale which is provided by a third scale portion on themeasurement structure and the first scale portion, the second scaleindicating the position of the measurement structure relative to thereference structure and so indicating the second distance.
 5. Themeasuring instrument of claim 4, in which the mask restricts the lengthof at least one of the first and third scale portions that is visible tothe user.
 6. The measuring instrument of claim 2, which includes (a) afirst measurement structure for measuring the first distance between thereference point and the first measurement point, the first measurementstructure and the reference structure being capable of movement relativeto one another, and (b) a second measurement structure for measuring thesecond distance between the reference point and a second measurementpoint, the second measurement structure and the reference structurebeing capable of movement relative to one another.
 7. The measuringinstrument of claim 6, which includes a plate member which has a firstface which is directed towards the reference structure and a second facewhich is directed away from the reference structure, the first faceforming part of one of the first and second measurement structures andthe second face forming part of the other of the first and secondmeasurement structures.
 8. The measuring instrument of claim 6, in whichthe first measurement structure and the second measurement structurehave components in common.
 9. The measuring instrument of claim 2, inwhich the reference structure includes a formation which can bepositioned against the reference point to enable the reference structureto be located relative to the reference point.
 10. The measuringinstrument of claim 9, which is for use in a knee replacement procedureto measure the size of a patient's femur, in which the formationcomprises a stylus for contacting the anterior cortex of the femur tolocate the reference structure relative to the anterior cortex.
 11. Themeasuring instrument of claim 2, in which the measurement structureincludes a component for fixing the measurement structure againstmovement relative to the bone in at least one degree of freedom.
 12. Themeasuring instrument of claim 11, which includes an intramedullary rod,and in which the fixing component can be connected to the intramedullaryrod.
 13. The measuring instrument of claim 11, in which the measurementstructure includes (a) a formation for locating the measurementstructure relative to the first measurement point or the secondmeasurement point, and (b) a driver for moving the formation relative tothe connector component.
 14. The measuring instrument of claim 11, inwhich the reference structure can be positioned on the measurementstructure relative to the reference point while the measurementstructure is connected against movement relative to the bone.
 15. Themeasuring instrument of claim 1, in which the mask can be fastenedagainst the second scale in a plurality of different positions, eachposition corresponding to a different size of the first measureddistance.
 16. The measuring instrument of claim 1, in which the mask isfastened against the second scale so that it can slide relative to thesecond scale to allow it to be placed in a selected positioncorresponding to the size of the first measured distance.
 17. Ameasuring instrument for use during an orthopaedic surgical procedure tomeasure first and second distances from a reference point on a bone tofirst and second measurement points respectively, which comprises: a. areference structure which can be located relative to the reference pointon the bone, b. a first measurement structure for measuring the firstdistance between the reference point and the first measurement point,the first measurement structure and the reference structure beingcapable of movement relative to one another, and the instrumentincluding a first scale provided by a first scale portion on thereference structure and a second scale portion on the first measurementstructure, the first scale indicating the position of the firstmeasurement structure relative to the reference structure and soindicating the first distance, c. a second measurement structure formeasuring the second distance between the reference point and a secondmeasurement point, the second measurement structure and the referencestructure being capable of movement relative to one another, theinstrument including a second scale which is provided by a third scaleportion on the second measurement structure and the first scale portion,the second scale indicating the position of the second measurementstructure relative to the reference structure and so indicating thesecond distance, d. a mask which can be located against the first scaleportion in a position that is selected according to the size of thefirst measurement, the mask restricting the length of at least one ofthe first and third scale portions that is visible.
 18. The measuringinstrument of claim 17, in which the first measurement structure and thesecond measurement structure have components in common.
 19. Themeasuring instrument of claim 18, which includes a plate member whichhas a first face which is directed towards the reference structure and asecond face which is directed away from the reference structure, thefirst face forming part of one of the first and second measurementstructures and the second face forming part of the other of the firstand second measurement structures.
 20. The measuring instrument of claim18, which includes a driver which can be operated to move the platemember between the first and second measurement points.
 21. Themeasuring instrument of claim 20, in which the driver includes aconnector by which can be fixed in at least one degree of freedomagainst movement relative to the bone, and an adjuster controlling thedistance between the connector and the plate member.
 22. The measuringinstrument of claim 17, in which the second scale portion and the thirdscale portion are provided on a common component.
 23. The measuringinstrument of claim 17, the instrument including a driver assemblyassociated with at least one of the first and second measurementstructures, the driver assembly including a connector by which thedriver assembly can be fixed in at least one degree of freedom againstmovement relative to the femur, and an adjuster for adjusting theposition of the measurement structure relative to the connector.
 24. Themeasuring instrument of claim 23 which is intended for measurement ofthe distal femur in a knee replacement procedure, which includes anintramedullary rod which can be fitted in the femoral intramedullarycanal, in which the connector can be connected to the intramedullaryrod.
 25. A kit for use in a surgical procedure to implant a knee jointprosthesis, which comprises: (a) a measuring instrument as claimed inclaim 1, in which the mask includes at least one posterior cut referencesurface, and (b) a femoral component trial which includes an anteriorlyfacing bone contacting surface on a posterior condyle, in which the saidbone contacting surface defines a posterior cut plane, in which theposterior cut reference surface when the mask is positioned on thesecond scale of the instrument appropriately having regard to themeasured first distance, lies in the posterior cut plane which isdefined by the femoral component trial when the femoral component trialis positioned on the resected femur.
 26. A kit for use in a surgicalprocedure to implant a knee joint prosthesis, which comprises: (a) ameasuring instrument as claimed in claim 17, in which the mask includesat least one posterior cut reference surface, and (b) a femoralcomponent trial which includes an anteriorly facing bone contactingsurface on a posterior condyle, in which the said bone contactingsurface defines a posterior cut plane, in which the posterior cutreference surface when the mask is positioned on the second scale of theinstrument appropriately having regard to the measured first distance,lies in the posterior cut plane which is defined by the femoralcomponent trial when the femoral component trial is positioned on theresected femur.